Quadrupole mass analyzer



July 22, 1969 P. M. UTHE QUADRUPOLE MASS ANALYZER 2 Sheets-Sheet 1 Filed Sept. 13, 1965 w mr NU WI. E WA H m M P 7 2 f 1 N m s 0 V n c 0 C E 4 v 0 R X 5 2 aw!" atrw ATTORNEY Fig. 2

July 22, 1969 P. M. UTHE 3,457,404

QUADRUPOLE MASS ANALYZER Filed Sept. 13, 1965 2 Sheets-Sheet 2 INVENTOR.

P. MICHAEL UTHE mud/Ham A E M ATTORNEY Unite US. Cl. 250-419 12 Claims ABSTRACT OF THE DISCLGSURE Sensitivity and resolution of a mass analyzer are increased by provision of an ion beam control aperture between the ion source and electrode structure said aperture having a diameter substantially equal to the distance from the axial center line of the electrode structure to the most adjacent surface of the electrodes.

This invention relates to a quadrupole mass analyzer and more particularly to controlling the ion beam size of the analyzer.

Quadrupole analyzers or spectrometers have been described in the prior literature and particularly in US. Patent No. 2,939,952. In such analyzers a controlled varying voltage is used in conjunction with a fixed frequency in performing a mass spectrum analysis. Such quadrupole analyzers may be used in measuring the composition of chemical substances and are comprised basically of an ionizer, a quadrupole section, and an ion detector. The chemical substance which is to be analyzed is introduced into the ionizer as a vapor at a low pressure comprised of neutral ions. A small proportion of the atoms or molecules which make up the chemical substance are ionized by electron bombardment and these ions are then accelerated and focused through an ion injection aperture int the quadrupole section in the form of an ion beam. The size of the aperture determines the cross-sectional size of the ion beam. Such ion beam is filtered by permitting only those ions within a specific range of charge-to-mass ratios to pass through the quadrupole section. Those ions which are able to pass through the quadrupole section are then collected by the ion detector such as an electron multiplier.

The output current produced by the ion detector is a measure of the number of atoms or molecules in the ion beam which have a particular charge-to-mass ratio. The specific charge-to-mass ratio which is detected is determined by the values of the voltages applied to the electrodes of the quadrupole section as described in detail in patent application Ser. No. 486,660, filed Sept. 13, 1965, for Electronic Compensation for Quadrupole Mass Analyzer, having the same applicant and assignee as the present invention. Since a majority of the ions which are ionized are singly charged, the mass number of the detected atoms or molecules may be directly determined from the output current of the detector and the value of the voltages applied to the quadrupole electrodes.

The important figures of merit of a quadrupole mass analyzer are sensitivity, resolution, and contamination. Sensitivity is defined as the number of ions that can be effectively employed by the analyzer per unit pressure in the analyzer. Accordingly, it is desirable to have the sensitivity as high a value as possible so that the signal to noise ratio is high as well as the speed of response. It is known that sensitivity of the analyzer increases as the ion beam current is increased. This increase in sensitivity occurs for the reason that the magnitude of the detected signal by the ion detector is directly proportional to the number of ions that enter the quadrupole section within a given length of time it it is assumed that all other parameters remain unchanged and that internal ion beam interactions are negligible. The contamination of an analyzer is the collection on the quadrupole electrodes of undesirable substances which decrease the sensitivity and resolution of the analyzer. Normally, contamination accumulates on the electrodes due to the residue from the ions being detected. Excessive contamination occurs when more ions than can be effectively employed by the analyzer are injected through the ion injection aperture into the quadrupole section. In this manner undesirable and rapid deterioration of analyzer performance occurs.

The resolution of the analyzer is defined as its ability to separate different masses that are being detected. The resolution of the analyzer is dependent upon many parameters an important one of which is the location of the injected ion beam with respect to the electric field produced about the quadrupole electrodes. The location of the ion beam entering the quadrupole section is important in known finite length quadrupole analyzers where such entrant ions should experience maximum radial acceleration at the earliest possible moment. For a high degree of resolution and for satisfactory operation of the analyzer it will be understood that those ions with charge-to-mass ratios leading to beam instability be removed so that the current measured by the ion detector be due only to those ions which have satisfied the stability criteria of the well known Mathieu-Hill equation:

where M=ion mass e=ion charge x=ion position r =the minimum distance from the zero field axis to the surface of an analyzer electrode V =app1ied direct current voltage V cos wt=applied alternating current voltage w=frequency t time In known analyzers the complete separation of unstable ions from stable ions is not possible as this would require a quadrupole section of infinite length and of perfect construction. However the earlier an ion in an ion beam can establish a stable trajectory the better the res- 01 ion of the analyzer. The relationship between the res olution and ion stability or degree of instability of the ion is not mathematically linear and decreases in importance as the number of oscillations experienced by the ion increases.

In order to maximize sensitivity and resolution with the least possible contamination prior analyzers have utilized an ion injection aperture between the ionizer and the quadrupole sections having as small an opening as possible with respect to electrode geometry. In this manner the ion beam has been formed with a relatively small cross-sectional area. However with such size injection aperture and ion beam it has been found that the sensitivity and resolution of the analyzer with respect to unit quadrupole electrode geometry was much less than can be obtained.

Accordingly, an object of the present invention is an ion injection aperture of a mass analyzer having an opening of size in which the sensitivity and resolution of the analyzer have been optimized without generating more contamination than necessary.

Another object of the present invention is a method of quadrupole analysis in which the ion beam size is controlled for the least contamination possible with maximized sensitivity and resolution.

In accordance with the present invention there is provided a quadrupole mass analyzer having an injection aperture which provides the opening from the ionizer to the quadrupole section and forms an ion beam having a predetermined cross section. The diameter of the injection aperture and thus the diameter of the cross section of the ion beam are made equal to r where r is measured from the axial center line of the analyzer section or zero field axis to the most adjacent surfaces of the analyzer electrodes. In this manner there is provided optimized sensitivity and resolution without generating more contamination than necessary.

In a preferred form of the invention a blocking grid is formed in the ion injection aperture so that the grid is aligned with the minimum field planes or regions of the analyzer. Specifically, the minimum field region is defined as the two planes that are at right angles which just fall between the electrodes or bisect the distance between the electrodes. No electric field exists in this minimum field region and thus the ion beam cannot be operated on in this region. Thus, resolution is increased by prohibiting ions from entering the quadrupole section in alignment with such zero field or zero potential region.

For further objects and advantages of the invention and for a typical embodiment thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings, in which:

FIGS. 1 and 1A illustrate in cross section the internal structure of a quadrupole mass analyzer embodying the invention;

FIG. 2 illustrates hyperbolic quadrupole electrodes connected to sources of supply;

FIG. 3 illustrates in detail the ion injection aperture of FIG. 1; and

FIGS. 4 and 5 illustrate modifications of the invention.

Referring now to FIG. 1 there is shown a quadrupole mass analyzer also known as a residual gas analyzer comprising a metallic chamber which is evacuated of air and having disposed therein quadrupole electrodes 11 in a quadrupole section 10b. The chemical substance to be analyzed is introduced into an ionizer section 16a by way of a molecular beam 20 comprising neutral ions before ionization. The ionizer section 1011 of the chamber 10 may be any one of the well known types and may comprise a filament 12, a grid 13, a cylindrical anode l5, and an ion extractor 17.

The substance 20 is introduced by way of the molecular beam as a vapor at a low pressure and a small percentage of the atoms or molecules which make up the chemical substance are ionized by electron bombardment within the ionizer 10a. These ions are then accelerated and focused by means of anode and ion extractor 17 through a circular ion injection aperture 21 formed in a flat metallic aperture plate 23 and into the quadrupole section 10b of the chamber 10. Injection aperture 21 forms a port or window which allows the ions to flow as an ion beam into the quadrupole section. Plate or member 23 is grounded and those ions produced by the ionizer and which strike the metallic aperture plate do not enter the quadrupole section and are removed from the ion beam. Thus, it will be understood that the ion injection aperture 21 forms the interface between the ionizer 10a and the quadrupole section 10b and the aperture size controls or determines the size of the circular cross-sectional ionbeam as illustrated in detail in FIG. 3.

The ion beam is filtered in the quadrupole section by permitting only those ions within a specific range of charge-to-mass ratios to pass through the quadrupole electrodes 11. The ions are separated in accordance with their stability and the ions that have stable paths are able to travel through the electric field provided about the electrodes 11 and such ions are collected by the ion detector 22.

The quadrupole or filter section of the analyzer shown in FIG. 1 is composed of four cylindrical metallic rods which are precisely located in a rectangular array as shown in FIG. 1A. The electrodes 11 are secured to the housing 10 by means of ceramic supports 1%. A DC. voltage is applied to each of the electrodes 11 and superimposed thereon an RF. voltage is also applied to electrodes 11 as described in detail in the above identified patent application. As a result of these applied voltages an electrostatic field is generated in the region between the electrodes and the ion beam is directed through the field and into the ion detector 22. The specific charge-tomass ratio of a particular ion determines whether such ion travels along a stable or unstable path.

Electrodes 11 may be hyperbolic cylinders 11a as shown in FIG. 2 rather than the circular cylinders 11 as shown in FIGS. 1 and 1A. In addition, the DC. voltage +V may be applied by way of a source 25 and conductors 27 to electrodes 11a and the RF. voltage +V cos wt may be applied by way of a source 26 and conductors 11a to electrodes 110. Thus, for the system of FIG. 2, the filtering force applied to an ion traveling through the quadrupole section may be described by the following equations:

It will be understood that F and F represent the x and y direction components of the force vector. For purposes of explanation and simplicity the phase angle relating to the oscillatory or cosine forcing functions have been omitted. The position coordinates x and y are defined for the purpose of this invention as being zero at the analyzer axial center line which is the common longitudinal axis of the electrodes 11. In addition coordinates x and y are defined as being equal to r, at the most adjacent surfaces of the electrodes 11a as shown in FIG. 2. Irrespec tive of the values of e, V V 4-: and r optimum filtering occurs when the applied force as above defined is maximized It is to be noted that no force occurs if ions enter the quadrupole section 1% at the position coordinates x equals zero and y equals zero and with the velocity components e equals zero and g7 equals zero. Thus, at these position coordinates and velocity components no filtering and therefore no analyzing occurs. In addition no filtering occurs in the minimum field regions which are defined by the two geometric planes described by x and y. It will be understood that these minimum field planes are at right angles with each other and each bisect the distance between the electrodes 11, 11a. Since no electrostatic field exists in the region of these planes the ion beam is not operated on and filtered in this region.

The maximum filtering occurs when x, y=r for the reason that in this region the electrostatic fieid provides maximum force and maximum acceleration. In the region x, y greater than r the ions are defined as removed from the quadrupole section 10b and such ions are therefore no longer involved in the analysis.

By solving Equations 1-3 on an analog computer it has been found that sensitivity and resolution can be maximized for minimum contamination of the electrodes by properly sizing the ion injection aperture 21. The criteria used in such optimization computation is that no ion satisfying the stability requirements of the analyzer should be allowed to leave the analyzer except at the detector 22. In addition the optimization criteria required that the ion beam current as generated by the ion source produce ions with uniform current density and maximized unidirectional velocity vectors. Accordingly, the ion entrance hole provided by the ion injection aperture 21 is required to be as large as possible with the further requirement that there be no unnecessary contamination of the electrodes. In accordance with the invention the optimum diameter of the circular injection aperture is substantially equal to 1' where r is measured from the axial center line of the quadrupole section to the most adjacent surface of each of the electrodes as shown in FIG. 3. With the aperresolution without generating more contamination than is necessary. It will be understood that the aperture 21 and thus the ion beam are not required to be of circular cross section but only to be of size having a cross-sectional dimension substantially equal to the value of the dimension r'.,. In other words, the cross section of the aperture or beam should have an area equal to a circle having a diameter equal to 11 In accordance with the invention the specific shape of the aperture is not the criteria but the optimum beam size and thus the aperture hole size must have a specific relative dimension.

While the ion injection aperture is shown in FIGS. 1 and 2 as a fiat aperture plate 23 with a circular opening it will be understood that the aperture may be formed in the shape of a cylinder 21a as shown in FIG. 4. The cylindrical aperture is eifective to focus the ion beam to improve the formation of a circular cross-sectional ion beam. In the manner previously described, in accordance with the invention the diameter of the cross section of the cylindrical opening is made substantially equal to r An ion injection aperture 21b of a plate 23b, FIG. 5, may also include a blocking grid 30. Blocking grid 30 has its grid material aligned with the minimum field planes of the quadrupole section 1011 which as previously defined such planes are described by x equals y. Since no field exists in the region of these minimum field planes grid 30 enhances resolution by prohibiting ions from entering the quadrupole section in alignment with these zero potential loci. Thus, the ion beam is not injected into these minimum field regions where the field cannot operate on the ion beam and in this manner the resolution is increased.

A typical example of a quadrupole mass analyzer according to the invention may have the following general characteristics:

analyzer electrode length=5l0 inches analyzer electrode spacing r =0.l25 inch analyzer voltage ratio V /V =0.168

ion source-electron bombardment type using 90 ev. electrons and generating a focused ev. ion beam detectorl6 stage Cu-Be multiplier operated at 2000 volts.

Modifications of this invention not described herein will be apparent to those skilled in the art and it is intended that the matter contained in the foregoing description be interpreted as illustrative and not limitative, the scope of the invention being defined in the appended claims.

What is claimed is:

1. A mass analyzer system for separating ions from an ion beam having different charge-to-mass ratios by causing the ions to assume oscillations which are a function of such charge-to-mass ratios comprising;

an ion source and an ion detector spaced axially from each other in a chamber,

at least four electrodes extending longitudinally within said chamber between said ion source and said ion detector, said electrodes being radially spaced with respect to an axial center line,

a D. C. voltage source and an R. F. voltage source each connected to each of said electrodes to provide an electrostatic field for filtering said ions in said ion beam, and

a member located in said chamber between said ion source and said electrode having an injection aperture for said ions centrally located on said axial center line providing an opening from said ion source to said electrodes to form an ion beam size having a cross-sectional dimension substantially equal to the distance between said center line and the most adjacent surface of an electrode.

2. The mass analyzer system of claim 1 in which said injection aperture has a substantially circular shape with a diameter substantially equal to said distance between said center line to the most adjacent surface of an electrode.

3. The mass analyzer system of claim 1 in which said injection aperture is cylindrically shaped having a crosssectional diameter substantially equal to said distance between said center line to the most adjacent surface of an electrode.

4. The mass analyzer system of claim 1 in which said injection aperture includes a grid having a plurality of metallic elements, each of said elements being aligned with respective ones of the minimum electric field planes of said electrodes which bisect the distance between said longitudinally extending electrodes, whereby said grid blocks a portion of said ions from said minimum field planes.

5. A quadrupole mass analyzer system for separating ions from an ion beam having different charge-to-mass ratios by causing the ions to assume oscillations which are a function of such charge-to-mass ratios having optimized sensitivity and resolution comprising an ion source and an ion detector spaced axially from each other in a vacuum chamber,

at least four electrodes extending longitudinally within said chamber between said ion source and said ion detector, said electrodes being radially spaced with respect to an axial center line,

a DC. voltage source and an RF. voltage source each connected to each of said electrodes to provide an electrostatic field for filtering said ions in said ion beam, and

said chamber including a member located between said ion source and said electrodes having an injection aperture for said ions aligned with respect to said axial center line providing an opening for said ion beam to said electrodes having a cross sectional area substantially equal to a circle having a diameter equal to the distance between said center line and the most adjacent surfaces of said electrodes.

6. A quadrupole mass analyzer system for separating ions having difiering charge-to-mass ratios by causing the ions to assume oscillations which are a function of such charge-to-mass ratios having optimum sensitivity and resolution comprising a source of ions and an ion detector spaced axially from each other in a vacuum chamber,

a plurality of elongated electrodes extending longitudinally between said source of ions and said ion detector and spaced radially with respect to a common longitudinal axis of said electrodes,

a DC. voltage source and an R.F. voltage source each connected to each of said electrodes to provide an electrostatic field for filtering said ions, and

said source of ions including an aperture member in said chamber located between said source and said electrodes having an injection aperture centrally located with respect to said common longitudinal axis for allowing said ions to flow between said electrodes as an ion beam having a diameter substantially equal to the distance between said common axis and the most adjacent surface of at least one electrode whereby sensitivity and resolution are optimized without generating more contamination on said electnodes than necessary.

7. The system of claim 6 in which said aperture member has a cylindrically shaped ion injection aperture having a cross-sectional diameter substantially equal to said distance between said common axis and the most adjacent surface of at least one of said electrodes.

8. The system of claim 6 in which said injection aperture includes a metallic blocking grid having a plurality of blocking elements, each of said elements being aligned with respective ones of the minimum field planes of said electrodes, each of which planes bisects the distance between said longitudinally extending electrodes thereby to increase resolution by prohibiting ions from said minimum fiield planes.

9. A mass analyzer system for separating ions having differing charge-to-mass ratios by causing the ions to assume oscillation which are a function of such charge-tomass ratios comprising:

an ion source and an ion detector spaced axially from each other in a vacuum chamber,

four electrodes extending longitudinally within said chamber and between said ion source and said ion detector, said electrodes being radially spaced with respect to an axial center line where the electric field force components developed by said electrodes equal zero,

a DC voltage source and an RF. voltage source each connected to each of said electrodes to provide an electrostatic field for filtering said ions,

said ion source including an opening aligned with said axial center line to allow ions into said electrodes having a cross-sectional dimension substantially equal to the distance between said center line and the most adjacent surfaces of said electrodes, and

said opening having formed therein a grid including a plurality of metallic elements, each of said elements being aligned with respective ones of the minimum field planes of said electrodes defined by X=Y to provide increased resolution where X equals a first vector force component of said electric field and Y equals a second vector force component separated by a right angle from said first component, whereby said grid increases resolution and sensitivity by prohibiting ion flow to said minimum field planes. 10. The method of separating ions having different charge-to-mass ratios which comprises forming an electric field between electrodes radially spaced with respect to an axial center line having a DC. component and an R.F. component,

forming said ions into an ion beam having a crosssectional dimension substantially equal to the distance between said center line and the most adjacent surfaces of said electrodes, and

passing the ion beam through the electric field.

11. The method of separating ions of differing chargeto-mass ratios which comprises forming an electric field between electrodes radially spaced with respect to a common longitudinal axis having a DC. component and an RF. component whereby ions flowing through the electric field are caused to assume oscillations which are a function of their charge-to-mass ratios,

fonming said ions into an ion beam of circular crosssection having a diameter substantially equal to the distance between said common longitudinal axis to the most adjacent surface of at least one of said electrodes, and

passing the ion beam through the electric field formed between the electrodes thereby to optimize sensitivity and resolution without generating more contamination than necessary.

12. In the method of claim 11 restraining ions from being injected into the minimum field regions within said electric field which regions bisect the distance between the electrodes.

References Cited UNITED STATES PATENTS 2,950,389 8/1960 Paul et al 25041.9 3,129,327 4/1964 Brubaker 250-419 3,280,326 10/ 1966 Gunther 250-419 WILLIAM F. LINDQUIST, Primary Examiner 

